1. Field of the Invention
The present invention relates to an automatic focusing device for use in a camera or the like.
2. Related Background Art
Most of the automatic focusing systems employed in single-lens reflex cameras achieve focusing by the repetition of a cycle of "focus detection (entry of sensor signal and focus calculation), and lens driving". The amount of lens movement in each cycle is based on the amount of defocus obtained in the focus state detecting operation in said cycle, and the lens movement is conducted in anticipation for eliminating the defocus found at the focus state detection, at the end of the lens movement.
The focus state detection and the lens movement naturally require a certain time. However, as the amount of defocus remains constant for a stationary object unless the lens is moved, the amount of defocus eliminated at the end of lens movement is equal to the amount of defocus detecting at the focus state detection, so that correct focusing can be achieved.
However, in case of an object with a large amount of movement, the amount of defocus may vary in the course of focus state detection and lens movement. Consequently the amount of defocus to be eliminated may be considerably different from the detected amount of defocus, so that the lens may not be focused on the object at the end of lens movement.
In Japanese Patent Application 62-263728 the assignee of the application proposed an automatic focusing method for resolving the above-mentioned drawback.
Said proposed method consists of approximately representing the relation between the position of an image plane resulting from the movement of the object and time with a first-order function and a second-order function in consideration of the amount of detected defocus and the amount of lens movement in each cycle and the intervals of said cycles, thereby correcting the amount of lens movement, and is expected to alleviate said drawback.
However such a focusing method based on prediction may result in an erroneous lens movement if errors are involved in the focus state detecting operation and in the lens driving operation.
Such erroneous lens movement resulting in such errors will be explained in the following.
FIG. 2 is a chart showing the above-mentioned method for correcting the lens movement, in which the position x of the image plane of the object in the ordinate varies as a function of time t in the abscissa.
The solid curve x(t) indicates the position of image plane, at a time t, of the object axially approaching to the camera, when the photographing lens is focused at an infinite distance. The broken-lined curve l(t) indicates the position of the photographing lens at a time t, and proper focusing is achieved when x(t) conicides with l(t). A period [t.sub.i, t.sub.i '] indicates the duration of focus state detection, while [t.sub.i ', t.sub.i+1 ] indicates the duration of lens movement. Also in the illustrated example it is assumed that the position of the image plane moves according to a second-order function (at.sup.2 +bt+c). Therefore, if three image plane positions (t.sub.1, x.sub.1), (t.sub.2, x.sub.2) and (t.sub.3, x.sub.3) at present and in the past can be known at a time t.sub.3, it is possible to predict the image plane position x.sub.4 at a time t.sub.4 which is later than t.sub.3 by an "AF time lag" and a "release time lag". The AF time lag is the time required for focus state detection and lens movement, while the release time lag indicates the time from the output of the shutter releasing instruction to the start of the exposure operation.
However, what is in fact detected by the camera is not the image plane positions x.sub.1, x.sub.2, x.sub.3 but the amounts of defocus DF.sub.1, DF.sub.2, DF.sub.3 and the amount of lens movement DL.sub.1, DL.sub.2 calculated from the amounts of image plane movement. Also the time t.sub.4 is a future value which is variable according to the change in accumulating time of an accumulating sensor depending on the luminocity of the object, but is assumed as follows for the purpose of simplicity: EQU t.sub.4 -t.sub.3 =TL=TM2+(release time lag) (1)
Based on this assumption, the amount of lens movement DL.sub.3 can be calculated from the result of focus state detection at t.sub.3 as follows: EQU x(t)=at.sup.2 +bt+c (2)
By taking (t.sub.1, l.sub.1) as the original point, there can be obtained: ##EQU1## Then a, b and c can be calculated as follows by substituting (3), (4), and (5) in (2): ##EQU2##
Consequently the amount of lens movement DL.sub.3 corresponding to the amount of image plane movement at the time t.sub.4 can be determined as: ##EQU3##
In the following there will be explained the error in prediction resulting from the errors in the focus state detection and lens movement.
FIG. 3 shows the relationship between the position of the image plane and time, wherein a solid line indicates the actually moving position of the image plane. This can be approximated by a second-order function: EQU x=at.sup.2 +bt+c (10)
On the other hand, the image plane position recognized by the camera, calculated from the detected amount of defocus and the amount of lens movement, involves an error because said amount of defocus and said amount of lens movement both contain certain errors. More specifically the amounts x.sub.1', x.sub.2 ' and x.sub.3 ' can be determined in the following manner: EQU x.sub.1 '=DF.sub.1 '=DF.sub.1 +.delta.f.sub.1 ( 11) EQU x.sub.2 '=DF.sub.2 '+DL.sub.1 '=DF.sub.2 +DL.sub.1 +.delta.f.sub.2 +.delta.l.sub.1 ( 12) EQU x.sub.3 '=DF.sub.3 '+DL.sub.1 'DL.sub.2 '=DF.sub.3 +DL.sub.1 +DL.sub.2 +.delta.f.sub.3 +.delta.l.sub.1 +.delta.l.sub.2 ( 13)
wherein DF.sub.1 ', DF.sub.2 ', DF.sub.3 ' are detected amounts of defocus; DL.sub.1 ', DL.sub.2 ' are amounts of lens movement detected by the camera; DF.sub.1, DF.sub.2, DF.sub.3 are true amounts of defocus; DL.sub.1, DL.sub.2 l are amounts of actually conducted lens movement; .delta.f.sub.1, .delta.f.sub.2, .delta.f.sub.3 are errors in the focus state detection; and .delta.l.sub.1, .delta.l.sub.2 are errors in the lens movement.
Consequently the differences .delta..sub.1, .delta..sub.2, .delta..sub.3 between the image plane positions recognized by the camera and the actual image plane positions are determined as follows: EQU .delta..sub.1 =x.sub.1 '-x.sub.1 =.delta.f.sub.1 ( 14) EQU .delta..sub.2 =x.sub.2 '-x.sub.2 =.delta.f.sub.2 +.delta.l.sub.1 ( 15) EQU .delta..sub.3 =x.sub.3 -x.sub.3 =.delta.f.sub.3 +.delta.l.sub.1 +.delta.l.sub.2 ( 16)
Also the second-order function passing through (t.sub.1, x.sub.1 '), (t.sub.2, x.sub.2 ') and (t.sub.3, x.sub.3 ') is represented by: EQU x=a't.sup.2 +b't+c' (17)
so that there will result an error .delta.p in prediction between the lens position x.sub.4 ' determined from the above-mentioned function and the image plane position x.sub.4 at t.sub.4.
Said prediction error .delta.p is often several times as large as the error in the focus state detection or the error in the lens movement.
As explained in the foregoing, the error in the focus state detection or in the lens movement, which is negligible in the conventional auto focusing device will be amplified several times and will become intolerable in the predicting-type automatic focusing device in which following correction is conducted.